1. Field of the Invention
The present invention relates to a semiconductor device having a circuit structured with a thin film transistor. For example, it relates to the structure of an electro-optical device, typically a liquid crystal display device, and of an electric equipment loaded with such an electro-optical device as a component. Note that throughout this specification, the semiconductor device indicates general devices that may function by use of semiconductor characteristics, and that the above stated electro-optical device and electric equipment are categorized as the semiconductor device.
2. Description of the Related Art
In recent years, the technique of crystallizing and improving the crystallinity of an amorphous semiconductor film or a crystalline semiconductor film (a semiconductor film having crystallinity which is polycrystalline or microcrystalline, but is not a single crystal), in other words a non-single crystal semiconductor film, formed on an insulating substrate such as a glass, has been widely researched. Silicon film is often used as the above semiconductor film.
Comparing a glass substrate with a quartz substrate, which is often used conventionally, the glass substrate has the advantages of low cost and good workability, and can be easily formed into a large surface area substrate. This is why the above research is performed. In addition, the reason for preferably using a laser for crystallization is that the melting point of a glass substrate is low. High energy can be imparted to a non-single crystal film be means of a laser without causing much change in the temperature of the substrate.
A crystalline silicon film formed by performing laser annealing has a high mobility. Accordingly, it is actively used in monolithic type liquid crystal electro-optical devices, where thin film transistors (TFTs) are formed using this crystalline silicon film, for example, TFTs for driving pixels and for driver circuits, are formed on one glass substrate. The crystalline silicon film is formed from many crystal grains. Therefore, it is called a polycrystal silicon film or a polycrystal semiconductor film.
Further, a method of performing laser annealing by processing a high output pulse laser beam, such as an excimer laser by means of optical system, into a square spot of several centimeters, or into a linear shape with a length of 10 cm or more, on the surface to be irradiated, and scanning the laser beam (the laser beam irradiation position is moved relatively to the surface to be irradiated), has been preferably used because it has good mass productivity and is superior industrially. In addition, continuous emission lasers with very high output, such as an Ar laser, have been recently developed. There are reports of good results obtained when using a continuous emission laser for annealing a semiconductor film.
In particular, if a linear shape laser beam is used, then a high degree of mass productivity can be obtained because unlike the case of using a spot shape laser beam with which it is necessary to scan forward, back, left, and right, laser irradiation can be performed over the entire surface to be irradiated by scanning only at a right angle to the longitudinal direction of the linear shape laser. This is because scanning at a right angle to the longitudinal direction is the most efficient scanning direction. Due to this high mass productivity, the present use in laser annealing of a linear shape laser beam in which a pulse emission excimer laser beam is processed into a suitable optical system, is becoming a mainstream.
For the case of processing the above pulse emission excimer laser beam into a linear shape and irradiating the linear shape laser beam while scanning, for example, with a non-single crystal silicon film, the phenomenon of stripes at a portion where the beams overlap is noticeable. (Refer to FIG. 22A.)
The semiconductor characteristics of the film differ remarkably for each of these stripes, so if this striped film is used when forming an integrated driver and pixel (system on panel) liquid crystal display device, a drawback develops where these stripes appear on the screen, as is. The stripes which appear on the screen are caused by the non-uniform crystallinity in both the driver section and the pixel portion. This problem is being remedied by improving the film quality of the non-single crystal silicon film, the laser irradiation object, but this is not yet enough.